Most electronic and computer systems require DC voltage for their operation. Many systems further require more than one DC voltage level. A typical system has a power supply which connects to a standard 110 volt or 220 volt AC line and then converts the AC to one or more DC voltage levels. In order to ensure an uninterrupted supply of power, some systems employ a battery at the primary DC output of the power supply to hold up its level in the event of a loss of AC power. Secondary or auxiliary DC outputs of the power supply do not normally have a battery to hold up their level in the event of a power interruption but receive power from the battery at the primary DC output when AC power fails.
An auxiliary DC output voltage may be provided by a DC to DC converter which may be either an up-switcher or down-switcher circuit depending upon the level of voltage desired at the auxiliary output. Many converters have active electronic elements which require a supply voltage to operate. For example, a converter may employ a pulse width modulator chip which requires a minimum supply voltage to operate.
It is desirable to provide a supply voltage to the converter whenever the power supply is receiving AC power. Furthermore, it is desirable to maintain this supply voltage in the event of an interruption of AC power. However, maintenance of a converter supply voltage must stop if excessive battery discharge would result. Other conditions where it is desirable to discontinue providing supply voltage to the converter when AC power is not available include an improperly installed battery at the DC output and receipt of a control signal from the system.